Electrical compensation of optical impairments

Abstract

Optical impairments such as dispersion and fibre nonlinearity are compensated by generating a pre-distorted electrical signal at the transmitter. This signal is modulated onto a carrier signal, so that it is upconverted in frequency. This up converted signal is then used to modulate an optical source. Generally the optical signal will have two sidebands, one of which has the correctly pre-distorted information and the other which is unwanted. Information in the unwanted optical sideband is either filtered optically or electrically. In the preferred embodiments, the transmitter uses a tunable semiconductor laser with an integrated electroabsorption modulator to modulate the light. The preferred receiver is a coherent receiver with a tunable local oscillator laser. The receiver uses an electrical filter to remove the information in the unwanted sideband.

Description

FIELD OF THE INVENTION[0001]The present invention relates to a method and apparatus for compensating for optical impairments electrically and a system incorporating the same.BACKGROUND TO THE INVENTION[0002]Optical communications systems typically comprise of a transmitter which converts electrical signals into optical signals, an optical link over which the optical signals are transported and a receiver which converts the optical signal into an electrical signal, the electrical signal recovered at the receiver ideally being identical to the electrical signal originating at the transmitter. The optical link generally consists of optical transmission fibre to convey the signal and optical amplifiers to compensate for the loss that the fibre introduces. The optical amplifiers are generally interspersed at regular intervals along the link (e.g. every 40-200 km).[0003]One impairment the optical fibre may exhibit is chromatic dispersion. This is where different optical frequencies of the...

AI Technical Summary

Benefits of technology

The invention provides a pre-distortion unit for compensating optical transmission impairments in a communications system. This unit uses an electrical filter to modify an electrical input signal with a pre-distortion function to mitigate the impairments in an optical system. The pre-distortion function can be a complex function to achieve better performance. The invention also provides a system that combines the pre-distortion unit with an optical transmitter and an optical modulator to generate a pre-distorted optical signal for transmission through the optical communications system. The pre-distorted optical signal has the advantage of not needing a complex optical modulator and can be used with a low-cost optical modulator. The invention also provides methods for compensating dispersion, fibre nonlinearity, and modulator chirp in addition to dispersion or fibre nonlinearity. The receiver can have a local oscillator optical source or an interferometer for coherent interfering with the received signal. Overall, the invention provides a simple and cost-effective solution for compensating optical transmission impairments in a communications system.

Problems solved by technology

One impairment the optical fibre may exhibit is chromatic dispersion.

This results in the optical signal being distorted.

However, there are a number of drawbacks to using dispersion compensating fibre modules for compensating for the optical dispersion.

The most obvious disadvantage is that they tend to be very costly.

However, they are also relatively large and have high optical loss.

This latter feature results in more optical amplification being required which in turn increases cost and can have degrading effect on performance.

A further disadvantage is that dispersion compensating fibre modules tend to be more sensitive to nonlinear distortion, thus also reducing performance.

Such a modulator is considerably more expensive to fabricate than a conventional modulator, as the structure is larger and there are more processing steps involved in its fabrication.

Additionally a significant expense is the amplifier used to provide sufficient voltage swing to drive the amplifier.

Since two such amplifiers are required, this expense is increased.

A disadvantage of using a complex modulator is that its optical loss is generally significantly higher than a conventional one.

A further disadvantage is that the fabrication steps involved are very different from that of a semiconductor laser, such that it has not been possible to monolithically integrate a complex modulator with a semiconductor laser on the same substrate.

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